Mitsubishi Electric Mini-Split Code P4
Soft lockout
Drain sensor error / drain float switch connector open. The float switch connector CN4F on the indoor controller board is open, so the unit cannot confirm the condensate level is safe.
Manual wording: “Contact failure of drain float switch (CN4F) - Extract when the connector of drain float switch is disconnected.”
Documented for: A-Control indoor units with a drain float switch (PKA-A, PLA-A, PEAD-A, PCA-A)
Likely causes, most common first
- Contact failure of the float switch connector (insert failure)
- Float switch connector disconnected
- Defective indoor controller board
What you can safely check yourself
- Check the condensate drain line outside for a clog, kink or blockage
- Turn the system off at the breaker, wait one minute, and turn it back on once
When to call a technician
The float switch connector is inside the indoor unit on the control board. Never short or bypass the float switch to clear this code - it is the protection that keeps condensate from overflowing into the room.
A Mitsubishi Electric mini-split throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Indoor Unit Service Manual No. OCH761 Revised Edition-A - PKA-A LA Series (May 2021) (Mitsubishi Electric, PKA-A12LA, PKA-A18LA, PKA-A12LA.TH, PKA-A18LA.TH wall-mounted indoor units) · checked 2026-08-25